1. Field of the Invention
The present invention relates to image-forming apparatuses, such as copy machines and printers, and more specifically relates to an image-forming apparatus which uses a contact charge method and a transfer method and performs a toner-recycling process.
2. Description of the Related Art
A typical image-forming apparatus performs an electrophotography process including the basic steps of charging, exposure, development, transfer, and fixing. More specifically, a charging device uniformly charges the surface of a rotating photoconductor and an exposure optical system irradiates the charged surface of the rotating photoconductor so as to form an electrostatic latent image on the photoconductor. The electrostatic latent image moves toward a development section as the photoconductor rotates, and is developed by a development device so that a developer image (toner image) is formed on the photoconductor. Then, the toner image is conveyed to a transfer section and transferred onto a recording sheet by a transfer unit, and the recording sheet is conveyed toward a fixing device. At this time, some toner remains on the photoconductor after the transfer step, and moves toward a cleaning device as the photoconductor rotates. The toner remaining on the photoconductor after the transfer step is called residual toner, and is collected by the cleaning device to prepare for the next cycle.
One known charging device uses corona discharge to charge the photoconductor. However, this device operates at a high voltage, and therefore inevitably produces an unpleasant ozone smell. In addition, this device includes a corona discharge unit, which is relatively large and requires a high-voltage power source. Accordingly, charging devices have recently been put to practical use which include a conducting roller or a conducting brush and use a contact charging method in which the photoconductor is sufficiently charged at a lower voltage and with lower ozone production compared to those using corona discharge.
In the above-described charging devices using the contact charging method, the surface of the photoconductor is charged to a predetermined potential with a predetermined polarity by applying a voltage to a charging member which is in contact with the photoconductor.
In addition, in the above-described charging devices using the contact charging method, the surface of the photoconductor may also be charged by direct charge injection in which electric charge is directly injected into the photoconductor via the charging member. In direct charge injection, the surface of the photoconductor is charged without causing electric discharge, using a medium-resistance charging member, such as a charge roller, which is in contact with the surface of the photoconductor. In this method, ions are not generated, and adverse effects due to discharge products are avoided accordingly.
On the other hand, although the residual toner is collected by the cleaning device and processed as waste toner, the waste toner is preferably not generated in view of environmental preservation.
Accordingly, image-forming apparatuses have recently been put to practical use which are free from the above-described cleaning device and which perform a toner-recycling process by a “simultaneous development and cleaning” method in which the residual toner remaining on the surface of the photoconductor is collected and reused by a development device.
In the “simultaneous development and cleaning” method, the residual toner remaining on the photoconductor after the transfer step is collected in the development step of the subsequent cycle, that is, when the latent image formed on the photoconductor is developed, by applying a back bias (back potential difference Vback equal to the difference between the potential of a direct voltage applied to the development device and the potential on the surface of the photoconductor).
In this method, the residual toner remaining on the photoconductor after the transfer step is collected and reused in the following cycle so that the waste toner is not generated, and therefore the running cost of the printing operation is reduced. In addition, the cleaning device can be omitted and it is not necessary to provide a space for the cleaning device, so that the size of the image-forming apparatus is greatly reduced.
The inventors of the present invention have proposed image-forming apparatuses which perform the above-described toner-recycling process in Japanese Patent Laid-Open Nos. 10-307454, 10-307455, and 10-307456.
In these image-forming apparatuses, however, the charge roller often picks up substances like the residual toner adhering to the photoconductor since it is in contact with the surface of the photoconductor to charge the surface of the photoconductor, and therefore the charge roller is easily contaminated. When the charge roller is excessively contaminated, uniform charging cannot be achieved and the charging performance is degraded. In addition, in the above-described image-forming apparatuses which are free from the cleaning device and which perform the toner-recycling process (cleaner-less process), the residual toner and the like accumulates on the charge roller or in a region near a nip region between the photoconductor and the charge roller when the image-forming operation is continuously performed. This is because the residual toner has the same polarity as that of the charge roller and easily accumulates in an upstream region of the nip region in the rotating direction of the photoconductor.
When the residual toner accumulating in the nip region is discharged while an image is being formed, it will block exposure if it is in a black or halftone region of a graphic pattern or the like and causes an image defect. In addition, if the discharged toner is in a white region, it causes other kinds of image defects such as fogging.